A major consideration in the design of a mechanical support of a dual rotor machine is the size and therefore the cost of the bearings that support the two rotors relative to each other and relative to a supporting stator or housing. The problem is centred around the size of the bearing and the needed support structure that is necessary for inclusion between two of the rotors or between a rotor and the stator in the larger diameter context of the bearing being radially outwards of an inner bearing used to support the other of the rotors. The very much larger outer bearing is extremely expensive. The same difficulties apply irrespective of the exact nature of the machine that could, for example, be a motor, a generator, or a magnetic gearbox.
Bearing steel is typically the most expensive structural steel in any particular electrical machine. A conventional bearing arrangement includes two sets of large bore bearings that support the two rotors. Bearing bore diameters are limited to about 1200 millimeters and in large machines, this bearing bore limitation translates into large and heavy structures which are needed to reach and support the required rotor diameters apart from being highly costly.
Internally excited permanent magnet induction generators are known and function on the principle of having an additional, freely rotating permanent magnet rotor in combination with a normal induction rotor. The permanent magnet induction generator is generally positioned between the induction rotor and the stator. The permanent magnet rotor provides flux within the machine which alleviates the need for a magnetizing current and which, in turn, results in an improved power factor for the machine as a whole.
Most commercially available wind energy conversion systems currently utilize a combination of complex gearboxes and high speed induction machines. These systems are generally directly connected to an electricity grid, which is made possible by the induction machine being capable of slipping, thus allowing for a soft grid connection. Gearboxes and power converters used in conventional wind energy conversion systems are respectively mechanically and electronically expensive, maintenance intensive items of equipment which increase the overall cost of setting up and operating such a system. Gearboxes also contribute substantially to overall system mass and losses due to, for example, heat and noise. Power converters, on the other hand, are complex and expensive, electrically sensitive systems.
Applicant perceives a need for a machine having two coaxial rotors and a stator that can be used in many different applications.